1. Technical Field
The invention relates to the removal of constituents from highly purified liquids, such as ultra-pure water, with a bulk resistivity in excess of one megohm-centimeter.
2. Background Art
Highly purified and ultra-pure water is produced by a series of unit processes that progressively remove suspended and dissolved constituents. These processes, when operated properly, produce water that contains only trace amounts of contamination and that can be utilized in various industrial applications where highly purified water is critical to operation of systems and manufacturing of various products. This type of water quality is particularly important to, but not limited to, industries and research activities related to semiconductors, pharmaceuticals, photovoltaics, metal plating, power generation and nanotechnology.
As industries have increased operational efficiency and product quality, there has been an increase in requirements for higher quality liquids and analytical capabilities to measure the quality of high purity liquids.
Currently, the best available technologies for the removal of trace amounts of insoluble and soluble contamination in highly purified water is sub-micron membrane filtration or ultrafiltration. Membrane filtration and ultrafiltration have demonstrated the ability to remove particles as small as 10 nm and reject soluble material with a molecular weight cutoff of 13,000. However, membrane technology is reaching the limit of its capability, because smaller pores or membrane selectivity will make it difficult to pass water molecules with a diameter of 0.3 nm. Furthermore, the filter membrane material may deteriorate and shed particles from the physical impact of abrasive particles and the chemical attack by purified water. It may be noted that highly purified and ultra-pure liquids can be highly reactive and aggressive solvents.
It is particularly difficult for filters to control organic or biological particles because with large surface areas and extended service, bacteria may form significant colonies on the surface of the membranes, in voids (“dead legs”), low flow areas, and on the downstream side of the membranes (“grow through”). Once established on the product side of the membrane, the bacteria colonies will multiply and slough off randomly into the product stream, contaminating the highly purified liquids. Moreover, even biological particles that remain on the upstream side of the membrane eventually dissociate or disintegrate, contributing smaller particles that pass through the membranes, releasing organic contamination into the product stream. Finally, membrane filters can be cleaned only with cleaning solutions which are themselves likely to be contaminated, or with disassembly and back flushing or replacement which can introduce new contaminates. Thus, it can be appreciated that membrane filtration and ultrafiltration may actually exacerbate the problem of removing trace contamination from highly purified and ultra-pure liquids.
Currently, the best available technologies for the measurement of particles within high purity and ultrapure liquids are the optical particle counter (OPC) and slipstream collection of particles on sub-micron filter, in combination with scanning electron microscopy/energy dispersive X-ray spectroscopy (SEM/EDS).
The smallest particle size detected by the OPC is 40 nm and the detection limit is not likely to improve because of the fundamental limits of laser light wavelength and light scattering. Importantly, this size limitation is larger than the capability of membrane filtration or ultrafiltration, and therefore is only used to detect significant system upsets and not as a process control instrument.
As an alternative to OPC, a small (approximately 50 to 100 ml/min) stream of liquid can be filtered through a 25 mm membrane with 50, 100, or 200 nm pores to collect particles. This process will require significant time to collect an adequate amount of particles. For example, it takes approximately three weeks of slipstream filtration in an ultra-pure water system to collect an adequate number of particles. Once particles have been collected, the sample filter can be removed and evaluated by SEM/EDS to determine particle concentration, particle size distribution, and elemental composition of the particles. This analytical procedure can only effectively examine particles larger than 50 nm and is ineffective as a process control instrument because of the three-week lag time associated with the collection period. Further, due to the limitation of SEM/EDS, this analytical technique will not reliably evaluate the elemental composition of particles that are less than 100 nm.
The limitations of the current best practices for the purification and analytical evaluation of highly purified and ultra-pure liquids described above are problems for all industries utilizing these liquids for utility operations and manufacturing. This situation is particularly relevant in the semiconductor industry where critical particle size in advanced manufacture facilities is now 16 nm (half the 32 nm line width used to conduct electricity within semiconductor devices).